How do I keep lastmod, changefreq and priority columns?

Make sure the original sitemap declares those fields — converters can only output columns that exist in the source. Open the sitemap in a browser and check whether each <url> entry contains <lastmod>, <changefreq> and <priority> tags. If they're missing, no converter can produce them; the sitemap simply doesn't track that data. If they're present but missing from your CSV, the converter is probably stripping them — xmlsitemapmaker.com keeps every present field by default and only omits the column when no URL in the sitemap has a value for it. For sitemap-index files, each child sitemap might have different field coverage; we union them so the CSV includes every column at least one URL uses.